How about a tooltip or an error tip below that tells me why this button is disabled?
I found out later that normal usernames have suggestions that popup – existing accounts that have an API key don’t show up at all, making a truly confusing experience. When I first typed the user’s name, nothing showed up at all and had no suggestions like “user not found” or “user already has an api key (max 1)” or something.
It feels strange that I can’t assign more than 1 API key for different things, though - I thought that was the main point of API keys to keep scope very particular - feels more like a bug.
The “Save” button will be disabled until you fill in the “Description” field. Try filling in that field and see if you are then able to save the API key.